The invention relates to testing of electrical switches, including their make-and-break behavior when the switches are actuated extremely slowly. An example is a switch that is actuated in response to an aneroid transducer that senses gradual changes of altitude of an aircraft. Environmental control systems represent another field of use in which very slow actuation of switches occurs. The electrical tests include the detection of a type of fault that is peculiar to switches when they are actuated very slowly. The tests classify, mark and sort the switches into categories according to operating characteristics.
Many methods are presently used for testing electrical switches to determine their hysteresis and other characteristics including the time of the switch contacts's breaking, transferring, and remaking. To provide examples of some of the limitations that are overcome by the present invention, the following three types of prior methods and apparatus are briefly described.
One prior method is simply to mount a switch on a test bed having a large-drum micrometer as the actuating device, manually advance the micrometer, and observe the switching of the contacts by means of a light bulb connected with the switch. The switch plunger's positions at the circuit's make and break times are known from the positions of the micrometer.
One of its problems is that the manually-operated micrometer can accidentally drive the switch beyond its limits of travel, causing immediate or latent switch failure. Also, not enough information is provided about the contact change-over time of a double-throw switch, which is the time between opening of one contact and closing of another.
Moreover, the operator may fail to detect a switch fault known as a "deadbreak", which is an unwanted pause period during snapover of the switch. Deadbreak is a contact hangup in which there is force equilibrium (zero net force) on the movable element without contact pressure, and the contacts are hung up temporarily as a movable contact separates mechanically from one stationary contact and goes to another.
A second prior apparatus is an electro-mechanical device as described above, but in which the contacts are coupled to a linear variable-differential transformer device (LVDT). The actuating and response signals of the switch are recorded on an ink trace chart. The operator manually measures the length (duration) of the ink trace with a card (FIG. 6) having a graduated scale showing switch categories, and assigns the switch to an indicated category. The results are often inaccurate because of the measurement method and human error.
A third known type of equipment, which is used and offered by one of the switch manufacturers, automatically measures switch actuation and de-actuation forces and provides a thermal printout of the results. This machine does not, however, fulfill all of the needs of certain switch users. Although information is provided regarding electrical contact changeover, its emphasis is on operating forces during the contacts' make and break intervals. It does not provide electrical deadbreak information to the degree required by many industrial users of the switches, such as those who use the switches in situations in which they are actuated very slowly.